The most common and most likely answer to this crossword clue is the 7 letter word WINDIGO
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Buddy explains!
The straight part
We know that "Monster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monster = windigo.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WINDIGO mean?
WINDIGO noun- A hybrid fish derived from a male brook trout and a female lake trout
- A malevolent and violent cannibal spirit found in Anishinaabe, Ojibwe, and Cree mythology, which is said to inhabit the body of a living person and possess him or her to commit murder.